bin/tahoe: clean up global-vs-subcommand arguments like --node-directory

The new rules for "bin/tahoe ARG1.. SUBCOMMAND ARG2.." arg:

* --node-directory is only accepted in ARG1, not ARG2
* create-*/start/stop/restart accept --basedir in ARG2, or an explicit
  basedir argument
* only one of --node-directory/--basedir/explicit-basedir is accepted
* --quiet/--version is only accepted in ARG1, not ARG2

Closes #166
rename VDriveOptions to FilesystemOptions
CLI: put "[global-opts]" in all command synopses